The Janus Kinases (JAK) are a family of cytoplasmic protein tyrosine kinases including JAK1, JAK2, JAK3 and TYK2. Each of the JAK kinases is selective for the receptors of certain cytokines, though multiple JAK kinases may be affected by particular cytokine or signaling pathways, including the pathways for IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21.
Phosphorylated JAK kinases bind various Signal Transducer and Activator of Transcription (STAT) proteins. STAT proteins are DNA binding proteins activated by phosphorylation of tyrosine residues and function both as signaling molecules and transcription factors and bind to specific DNA sequences of promoters of cytokine-responsive genes.
Cytokines influence cell differentiation, proliferation, and activation. Cytokines modulate both inflammatory and immune responses.
Abnormal JAK/STAT signaling is observed in conditions, such as allergies, asthma, autoimmune diseases such as transplant rejection, rheumatoid arthritis, amyotrophic lateral sclerosis, and multiple sclerosis, as well as in solid and hematologic malignancies such as leukemia and lymphomas. Inhibitors of the JAK pathway, particularly JAK3, are thought to be therapeutically useful for treating such conditions.
Certain JAK inhibitors are disclosed in WO 2007/077949 and WO 2008/084861.